This invention relates to protection devices for power elements included in integrated circuits. More specifically, this invention relates to devices for protecting final power transistors included in integrated circuits against outgoing overcurrents and overvoltages that may originate from shorts, for example.
The protection devices are integrated to the circuit which includes the power element to be protected, and accordingly, it should be possible to fabricate these circuits using simple and economical technologies, without losing power which could restrict the dynamic range of the power element. In addition, such devices should be able to provide a reliable, high degree of protection.
A conventional type of protection device that fulfills these demands has a circuit construction which includes at least one active element thermally coupled to the power element to be protected. This active element is connected to a control circuit which operates to turn off the integrated circuit to which the power element belongs upon the active element sensing a dangerous temperature level, indicative of excessive power being dissipated due to a condition of overvoltage or overcurrent.
This conventional type of protection, while being effective and reliable, is unsuitable where the abnormal conditions of operation are only temporary, since, lacking any external action, the device would remain off anyway.
It is for this reason that protectors are most frequently used. Although protectors are more complicated circuit-wise, their cut-in threshold is tied to the level of electrical quantities, such as current and voltage, that are related to the element to be protected, rather than to the level of heat dissipation. The level of such quantities is regulated to suit the dissipated instantaneous power, without the integrated circuit having to be turned off.
Such protection devices usually include circuit means for sensing and processing the value of the current flowing through the power element and of the voltage across it, to produce, on a given threshold of values being exceeded, the activation of a threshold circuit. Therefore, the current value through the power element is brought down to a maximum value which corresponds to the threshold and is a function of the voltage value across the element.
The maximum value that the current flowing through the power transistor can have without problems, and on which the sizing of the protection circuit would be dependent, is set by the physical characteristics of the same.
In general, for economical reasons, the users of integrated circuits that include power elements would size the external heat sinks for such elements to meet normal operation requirements, since short periods of large heat dissipation are well tolerated.
However, in a condition of protracted shorting, there exists a risk of the integrated circuit being damaged, and the surrounding material overheating and perhaps catching fire, from the heat generated and not adequately dissipated to the outside.
On the other hand, it would not be convenient to have the maximum current level through the power element decreased by lowering the cut-in threshold of the protection, because this would needlessly restrict the dynamic range of the circuit under normal operating conditions.
An example of a protection circuit of the type just outlined, for a power element in an integrated circuit, is disclosed in U.S. Pat. No. 4,623,950 assigned to this Applicant and hereby incorporated by reference.
Since power transistors fail when their working point moves out of their Safe Operating Area (SOA), which is largely limited by the maximum power which can be dissipated, it is important to be able to accurately measure the instantaneous power being dissipated by the transistors, in order to ensure proper operation of the transistors and best utilization of their potential for dissipation. It is vitally important that this measurement be dependent on the smallest possible number of process parameters, and independent of temperature. The sensing circuit employed heretofore actually provides coarse power measurements. In fact, such circuits usually are affected by temperature and several process parameters, and also disallow full exploitation of the power transistors. A simple circuit, whose design allows for utmost reproducibility and least use of silicon area is also of importance.
The present invention provides a sensing circuit (or sensor) for sensing the instantaneous power which can be dissipated through a power transistor in an integrated circuit, while having a simple construction, using a minimal amount of integration area, and affording improved measuring accuracy over conventional sensing circuits, so that full use of the transistor dynamic range can be made.
A preferred embodiment of the present invention includes a sensing circuit for instantaneous power which is dissipated, as indicated above, which includes a power transistor of the MOS type connected between the output terminal of a power stage and ground. It further comprises a MOS transistor having its gate terminal connected to that of the power transistor, its source terminal connected to ground, and its drain terminal connected to a circuit node. This circuit node is coupled to the output terminal by means of a current mirror circuit, which includes a resistive element in its input leg. Connected to the circuit node is the base terminal of a bipolar transistor which is respectively connected, through a diode and a constant current generator, between the output terminal and ground.